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By Charles Rhodes, P.Eng., Ph.D.

This web page briefly explains what a Fast Neutron Reactor (FNR) is and how it works. A much more complete description of the science underlying FNRs is set out in the 2021 text by Peter Ottensmeyer titled: Neutrons At The Core. The key issue of thermal stability of FNRs is set out on the web page titled: FNR Reactivity.

The FNR described herein consists of a geometrically stable assembly of solid nuclear fuel rods sealed inside 0.50 inch outside diameter chrome steel fuel tubes. This assembly of fuel rods in fuel tubes is entirely immersed in a highly thermally conductive pool of liquid sodium.

The nuclear physics of a geometrically stable FNR fuel assembly passively maintains the average nuclear fuel temperature at a fixed above ambient temperature, which is typically chosen to be 460 degrees C. When the liquid sodium temperature is less than the nuclear fuel temperature heat flows by thermal conduction from the nuclear fuel to the liquid sodium until the liquid sodium temperature adjacent to the fuel becomes equal to the fuel temperture.

Within the liquid sodium heat moves rapidly by both thermal conduction and thermal convection. Hence, at times when no heat is being extracted from the liquid sodium, the liquid sodium temperature quickly approaches the average fuel temperature, at which time the nuclear heat production stops.

If heat is continuously extracted from the liquid sodium the liquid sodium temperature adjacent to the fuel will be consistently less than the nuclear fuel surface temperature, causing heat to continuously flow from the nuclear fuel to the liquid sodium.

In general, if heat is extracted from the sodium pool the nuclear fuel will add an equal amount of nuclear heat to the sodium pool to keep the average nuclear fuel temperature constant. If heat extraction from the sodium pool stops the nuclear heat production also stops. This reactor power control process is entirely passive. There are no mechanical moving parts.

As long as the geometry of the solid fuel assembly remains constant, the sodium pool temperature will remain close to the chosen average nuclear fuel temperature, independent of the rate at which heat is extracted from the sodium pool.

As the name suggests a Fast Neutron Reactor (FNR) operates using the fast neutrons directly emitted by nuclear fission. These neutrons have average kinetic energies of about 2 MeV. A FNR is fundamentally different from a water cooled reactor which operates using slow (thermal) neutrons. The use of fast neutrons gives a FNR major advantages in terms of public safety, natural uranium utilization efficiency and elimination of long lived nuclear waste.

A practical FNR consists of a pancake shaped inner core containing fissile fuel that is completely surrounded by a thick neutron absorbing blanket containing fertile fuel. The fuel rods are contained in sealed vertical metal fuel tubes. The fission chain reaction occurs primarily in the core. Excess neutrons originating in the core are absorbed by U-238 in the blanket.

The blanket vertical thicknesses is set by the blanket fuel rod length. The core zone vertical thickness, and hence the gross reactor reactivity, are set by the core fuel rod length and by the amount of movable fuel bundle insertion into the matrix of fixed fuel bundles.

An important issue in FNR design is neutron conservation. Almost all the excess neutrons emitted by the core should be captured by the surrounding breeding blanket.


For simplicity, in the above diagram the air locks, the open steel lattice supporting the fuel bundles and the steel columns supporting the intermediate heat exchange bundles are not shown.

A modular liquid sodium cooled FNR, such as is described on this web site, consists of a 20 m inside diameter X 15 m deep sodium pool in which are immersed fuel bundles and intermediate heat exchange bundles. Outside the pool are factory fabricated and tested support modules for heat transport and electricity generation. The fuel bundles and the support modules are individually road truck portable, both before and after use. The primary sodium pool walls and the intermediate heat exchange bundles are protected from neutron impingment and do not become radioactive.

The FNR fuel assembly described on this web site has both core and blanket zones. The core zone contains Pu-239 and U-238 atoms and is where the nuclear chain reaction takes place. The blanket zone surrounds the core zone with U-238 atoms that absorb fission neutrons which escape from the core zone. Excess fission neutrons are also absorbed by U-238 atoms in the core fuel rods. After a short delay the resulting U-239 atoms spontaneously transmute through Np-239 to form new Pu-239 and later Pu-240 atoms. These plutonium atoms are then either immediately fissioned in the core or are later recovered from the blanket via fuel rod reprocessing and are used to make new FNR core fuel rods. In this process the resulting Pu-240 concentration is sufficiently high that the plutonium produced cannot be used to make practical nuclear weapons.

The FNR fuel assembly described herein consists of a disk shaped ~ 12.6 m diameter X ~ 0.4 m thick core which is surrounded by a 1.8 m thick blanket. The core contains a mixture of both fissile (potentially neutron emitting Pu-239 atoms and fertile neutron absorbing U-238 atoms. The blanket initially contains only fertile neutron absorbing U-238 isotope atoms.

The resulting disk shaped fuel assembly is mounted, disk axis upright, in the middle of a 20 m diameter X 15 m deep pool of liquid sodium and is supported 3 m above the primary sodium pool bottom by an open steel lattice structure. There are many thousands of small (~ 10 mm diameter) flow channels which pass through both the blanket and the core to allow liquid sodium to flow vertically through the fuel assembly for heat removal. The maximum safe thermal output of a FNR is limited by the properties of its fuel tubes.

The reactivity of the fuel assembly has a negative temperature coefficient. With no thermal load the reactivity is zero at a particular temperature, which is the reactor temperature setpoint.

There is a facility for occasional fine adustment of the reactor geometry while the reactor is operating. This facilty to enables fine adjustment of the FNR temperature setpoint.

If there is a step increase in reactor thermal load the fuel temperature briefly decreases which causes an increase in the reactor reactivity. In response the reactor thermal output power increases. When the increase in reactor thermal output power is sufficient to balance the increase in thermal load the reactor reactivity returns to zero. By this means the reactor keeps its fuel and hence the primary sodium pool at a nearly constant temperature.

There is a gadolinium skirt and a 1.7 m wide liquid sodium guard band surrounding the entire fuel assembly that absorbs any neutrons that escape from the fuel assembly to prevent these escaping neutrons causing cumulative neutron excitation and long term physical damage to the sodium pool structure and to the intermediate heat exchange bundles that are immersed in the liquid sodium adjacent to the pool side walls.

The fissile atom Pu-239 has the property that if it captures a free neutron it usually fissions and emits an average of 3.1 free neutrons per fission. Each fission reaction also emits about 200 MeV of heat energy.

If the FNR core zone is too thin or the fissile atom concentration in the core zone is too low the probability of a neutron that is emitted by one fissile atom being captured by another fissile atom before that neutron is absorbed by a non-fissile atom is less than (1 / 3.1), so no chain reaction is possible and the free neutron concentration in the core zone and the rate of fission heat production remain close to zero. However, if the core zone thickness is gradually increased while the fissile atom concentration in the core is sufficiently large the probability of a neutron emitted by one fissile atom being captured by another fissile atom eventually reaches (1 / 3.1). At this point a nuclear chain reaction commences and the concentration of free neutrons and the consequent rate of heat production in the core zone both rapidly rise.

The nuclear heat production in the core fuel causes the core fuel temperature to rise which, via thermal expansion, increases the average interatomic distance between the fissionable atoms. This thermal expansion reduces the rate at which neutrons are captured by Pu-239 atoms in the core zone and hence increases the rate at which neutrons diffuse out of the core zone into the blanket zone. The net result is that the temperature To at which the FNR reactivity is zero is fuel geometry dependent. At nuclear fuel temperatures less than To the free neutron concentration rises which produces more heat in the core zone fuel which causes a core zone temperature rise. At core zone temperatures above To the free neutron concentration in the core zone falls which stops heat production and hence stops the core zone fuel temperature rise.

Thus the average core zone fuel temperature is a function of the core zone geometry. As long as:
To > ambient temperature
as the core zone thickness increases so also does To. As the core zone thickness decreases so also does To.

As the fissile atom concentration in the core zone decreases so also does the core zone temperature. In the power FNR described herein as fissile fuel is consumed the core zone thickness must be periodically mechanically increased so that as the fissile atom concentration decreases over time the average core zone fuel temperature To remains constant.

When this fuel assembly is immersed in cool liquid sodium that liquid sodium warms, thermally expands and naturally rises through the vertical coolant channels in the fuel assembly. At its discharge from the top of the fuel assembly its average temperature is about 460 degrees C. This high temperature liquid sodium then flows radially outward across the top of the liquid sodium pool and then down through intermediate heat exchange bundles, where it cools down to about 410 degrees C, before flowing back radially toward the bottom center of the primary sodium pool. The heat removed from the flowing liquid sodium by the intermediate heat exchange bundles is transported to another building where it is used to make steam for electricity generation.

When the reactor thermal load is at its rated maximum the peak core fuel rod centerline temperature is 510 degrees C.
At full load there is an ~ 53 degree C temperature difference between the core fuel rod center line and the core fuel rod outside surface. Hence at full load the peak fuel rod surface temperature is:
510 C - 53 C = 457 C

Allowing for a 8 degrees C temperature drop across the fuel tube wall, at full load the primary liquid sodium discharge temperature is:
457 C - 8 C = 449 C.

However, due to nonuniformity is fuel rod fissile concentration and cooling channel open cross sectional area the average top of primary sodium temperature is set at about 460 degrees C.

The minimum permitted primary liquid sodium temperature at the fuel tube inlets is 410 C.

The maximum permitted primary sodium temperature rise is:
460 C - 410 C = 50 C

The typical full load sodium discharge temperature is:
Tpd = 460 C

Heat is removed from the intermediate heat exchange bundles via isolated NaK loops. The FNR thermal power output is controlled by controlling the NaK flow rate.

If there is no thermal load and no fission product decay heat the primary liquid sodium pool temperature will gradually rise to about 460 degrees C and then stop.

If due to nuclear heat release the temperature of the materials increases, thermal expansion of the materials in three dimensions increases the fraction of fission neutrons diffusing from the core zone into the blanket zone and decreases the rate at which Pu atoms in the core zone capture neutrons and then fission. Hence the free neutron concentration in the core falls and the nuclear chain reaction stops. Similarly, if the core temperature decreases the chain reaction restarts. By this mechanism the FNR maintains a nearly constant average fuel temperature in the core zone.

At full reactor power the primary liquid sodium pool is typically at 458 degrees C in the top 6 m of sodium, where there is thermal stratification and at 340 degrees C below that. As the reactor is unloaded the elevation at which this temperature transition occurs sinks by about 3.8 m. Hence, during a switch from reactor at zero power to reactor at maximum power the liquid sodium tempeature transition level rises by about 3.8 m.

If the chain reaction is initially off due to a high sodium pool temperature the amount of heat that must be extracted from the primary liquid sodium to bring the reactor to full power is:
3.8 m X Pi (10 m)^2 X 927 kg / m^3 X 50 deg C X 1.27 KJ / kg-deg C X 1 KWt-s / kJ
= 70,272,746 KWt-s
= 70,272 MWt-s

Assume that the power transition is caused by a 1000 MWt load step. Then the time for the FNR to fully change state in response to this load step is:
70,272 MWt-s / 1000 MWt
= 70.3 seconds

Provided that the fuel geometry remains stable and the primary sodium remains clean and is completely isolated from both air and water this energy production process, which involves no moving mechanical parts and no ongoing chemical changes, is extremely stable. One nuclear fuel load can power the reactor for about 30 years during which time about 15% of the reactor core fuel mass is converted into short lived fission products. In order to expedite fuel reprocessing a 20% fuel change is contemplated every six years.

The reactor normally operates with a fixed insertion setting for each moveable fuel bundle. In the core zone the rate of loss of Pu by fissioning is partially offset by the rate of production of Pu via neutron capture by U-238. Reactor criticality at the desired operating temperature is maintained through the operating life of fuel bundles via periodic small incremental changes in core zone thickness. These core zone geometry changes are accomplished by using liquid sodium hydraulic actuators to slightly change the vertical overlap between movable fuel bundles and their corresponding adjacent fixed fuel bundles.

During normal FNR operation most of the surplus neutrons are absorbed by U-238 isotope atoms which naturally transmute into Pu-239 isotope and Pu-240 isotope atoms. After a reactor fuel change the removed core and blanket fuel rods should be reprocessed to extract the plutonium atoms, which should then be used to make new core fuel rods. The Pu-240 isotope formation slightly reduces the Pu-239 isotope output but has the benefit that it makes the reactor generated plutonium unsuitable for military use.

If there is any unanticipated problem the reactor defaults into a safe walk away state with the sodium pool at it last temperature setpint.

A simple analysis of FNR economics is as follows. Consider an Nth of a kind, not a First Of A Kind (FOAK) FNR Nuclear Power Plant (NPP):
1) A 1000 MWt (300 MWe) FNR type nuclear power plant requires about 20,000 tonnes of material. The NPP selling price is $1.5 billion or:
1,500 million dollars / 300 MWe = $5 /We = $5,000 / kWe. Of that amount about $1000 / kWe must be reserved for material purchases. Hence the average cost of materials must be under:
($5,000 /kWe X 300,000 kWe X 0.9 X 0.2) / 20,000 tonnes
= $13,500 / tonne
$13.5 / kg

In order for this plant to have a simple payback period of 5 years exclusive of operating and maintenance costs the electricity that it produces must be sold for:
$1,500 million / 5 years X 300 MWe X 0.9 X 8766 h / year
= $ 1,000,000 /(year MWe X 0.9 X 8766 h)
= $1000 / (year kWe X 0.9 X 8766)
= $.1268 / kWhe

With operating, maintenance and electricity distribution costs this number becomes a retail price of about:
$0.20 / kWhe

Until the public grasps that this cost is unavoidable, fossil fuel displacement by sustainable clean nuclear power will go nowhere.

From a public energy cost perspective there is merit in energy savings that do not result in increases in fossil fuel consumption simply to mitigate the total ongoing energy cost.

This web page last updated August 15, 2022.

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